In recent years, semiconductor devices have become smaller in size and structures of semiconductor elements have become more complicated. In addition, the number of layers in multilayer interconnects used for a logical system has been increased. Accordingly, irregularities on a surface of a semiconductor device become increased, and hence step heights on the surface of the semiconductor device tend to be larger. This is because, in a manufacturing process of a semiconductor device, a thin film is formed on a semiconductor device, then micromachining processes, such as patterning or forming holes, are performed on the semiconductor device, and these processes are repeated many times to form subsequent thin films on the semiconductor device.
When the number of irregularities on a surface of a semiconductor device is increased, a thickness of a thin film formed on a portion having a step tends to be small. Further, an open circuit is caused by disconnection of interconnects, or a short circuit is caused by insufficient insulation between interconnect layers. As a result, good products cannot be obtained, and the yield tends to be reduced. Furthermore, even if a semiconductor device initially works normally, reliability of the semiconductor device is lowered after a long-term use. At the time of exposure in a lithography process, if a surface to be irradiated has irregularities, then a lens unit in an exposure system cannot focus on such irregularities. Therefore, if the irregularities of the surface of the semiconductor device are increased, then it becomes difficult to form a fine pattern on the semiconductor device.
Accordingly, in a manufacturing process of a semiconductor device, it becomes increasingly important to planarize a surface of a semiconductor device. The most important one of the planarizing technologies is CMP (Chemical Mechanical Polishing). The chemical mechanical polishing is performed with use of a polishing apparatus. Specifically, a substrate such as a semiconductor wafer is brought into sliding contact with a polishing surface such as a polishing pad while a polishing liquid containing abrasive particles such as silica (SiO2) is supplied onto the polishing surface, so that the substrate is polished.
This type of polishing apparatus comprises a polishing table having a polishing surface constituted by a polishing pad, and a substrate holding apparatus, which is called as a top ring or a carrier head, for holding a semiconductor wafer. A semiconductor wafer is polished by the polishing apparatus as follows: The semiconductor wafer is held by the substrate holding apparatus and then pressed against the polishing table under a predetermined pressure. At this time, the polishing table and the substrate holding apparatus are moved relative to each other for thereby bringing the semiconductor wafer into sliding contact with the polishing surface. Accordingly, the surface of the semiconductor wafer is polished to a flat mirror finish.
In such a polishing apparatus, if a relative pressing force between the semiconductor wafer being polished and the polishing surface of the polishing pad is not uniform over an entire surface of the semiconductor wafer, then the semiconductor wafer may insufficiently be polished or may excessively be polished at some portions depending on the pressing force applied to those portions of the semiconductor wafer. In order to avoid such a drawback, it has been attempted to form a surface, for holding a semiconductor wafer, of a substrate holding apparatus with use of an elastic membrane made of an elastic material such as rubber and apply a fluid pressure such as an air pressure to a backside surface of the elastic membrane so as to uniform a pressing force applied to the semiconductor wafer over an entire surface of the semiconductor wafer.
The polishing pad is so elastic that the pressing force applied to a peripheral portion of the semiconductor wafer tends to become non-uniform. Accordingly, only the peripheral portion of the semiconductor wafer may excessively be polished, which is called as “edge rounding”. In order to prevent such edge rounding, it has been used a substrate holding apparatus in which a semiconductor wafer is held at its peripheral portion by a guide ring or a retainer ring, and the annular portion of the polishing surface that corresponds to the peripheral portion of the semiconductor wafer is pressed by the guide ring or retainer ring.
Generally, a thin film formed on a surface of a semiconductor wafer has different film thicknesses at different radial positions due to the characteristics of a method and apparatus used to form the film. Specifically, the thin film has a thickness distribution in the radial direction of the semiconductor wafer. There has been known a polishing apparatus whose substrate holding apparatus has an adjustment mechanism for adjusting pressing forces applied to a polishing surface of a polishing table, as disclosed in Japanese laid-open patent publication No. 2003-106805 and Japanese laid-open patent publication No. 2002-187060. In this kind of polishing apparatus, the substrate, to be brought into sliding contact with the polishing surface, is divided into several zones, so that the pressing forces applied to the zones of the polishing surface are adjusted by the adjustment mechanism, respectively. According to the above-mentioned polishing apparatus, it is possible to adjust the pressing force distribution in the radial direction, and hence a uniform distribution of the film thicknesses can be achieved over the entire surface of the semiconductor wafer.
However, the film thickness distribution on the surface of the semiconductor wafer varies depending on the types of method and apparatus used to form the film. Specifically, radial positions and the number of thick portions, and a difference in thickness between the thick portion and thin portion vary depending on the types of method and apparatus used to form the film. Therefore, there has been a demand to provide a substrate polishing apparatus and a substrate polishing method which can cope with various substrates having various film thickness distributions and can polish the substrates easily at a low cost, rather than a substrate polishing apparatus which can cope with only a certain substrate having a certain film thickness distribution.